Embodiments of the present disclosure relate to a bell nipple for use in an offshore wellbore operation and pertain more particularly to a bell nipple comprising a piston for the purpose of passive heave compensation.
A hole is often drilled in the ground for the extraction of a natural resource such as ground water, brine, natural gas, or petroleum. In offshore drilling, a wellbore is drilled through the seabed in order to explore for and subsequently extract petroleum, which lies in rock formations beneath the seabed.
Offshore drilling is carried out from a floating rig, such as a drill ship, a semi-submersible platform, a floating drilling platform or a production platform. Similar to onshore drilling, a drill bit is suspended in the wellbore being drilled on a drillstring and rotated against the bottom of the wellbore in order to remove rock formations beneath the seabed and extend the wellbore.
During drilling of a subsea wellbore from a floating rig, a marine riser is used to return the drilling fluid, which is also called drilling mud, and rock cuttings from the wellbore to the surface. The riser is a pipe that extends from the drilling platform down to the seafloor. Drilling mud and cuttings from the borehole are returned to the surface through the riser annulus, which is an annulus formed between the drillpipe within the riser and the riser. The top of the riser is attached to the floating rig, while its bottom is secured to the wellhead at the seafloor. A blowout preventer (BOP) placed at the seafloor between the wellhead and the riser provides protection against over-pressured formations and sudden release of gas.
The top section of a riser is provided by a bell nipple. A bell nipple is an enlarged pipe at the top of a riser that serves as a funnel to guide drilling tools into the top of a well. The bell nipple is usually fitted with a side outlet to permit drilling fluids that have been circulated through the wellbore to flow to the mud treating equipment at the surface through another inclined pipe called a flowline.
The flowline is a metal pipe that connects the bell nipple under the rotary table to the possum belly at the mud tanks. The flowline is simply an inclined, gravity-flow conduit to direct mud coming out the top of the wellbore to the mud surface-treating equipment. When drilling certain highly reactive clays, the flowline may become plugged and require considerable effort by the rig crew to keep it open and flowing. In addition, the flowline is usually fitted with a crude paddle-type flow-measuring device commonly called a “flow show” that may give the driller the first indication that the well is flowing and/or a rate of flow.
The flowline may also be fitted with a flowmeter, such as a coriolis flowmeter or the like. A coriolis flow meter is a mass flow meter, also known as an inertial flow meter. The coriolis flow meter is a device that measures how much liquid is flowing through a tube. It does not measure the volume of the liquid passing through the tube, but measures the amount of mass flowing through the device.
The measurement of the flow rate of drilling fluids is very important for safe and effective drilling and logging operations. Accurately measuring the balance of the drilling fluids as a system, i.e. barrels-in versus barrels-out, provides important information to the driller and the mud logger, for example: early warning kick detection (a kick occurs when formation fluids flow into the wellbore and up the drill string due to imbalance of hydrostatic pressures); tracking ballooning and allowing the driller to differentiate between ballooning and kick; and accurate mud flow rate which is used by mud loggers to compute transport velocity and lag time, which is the time it takes for the mud to circulate from the drill bit to the surface.
Floating oil rigs such as drill ships and semi-submersible platforms are subjected to wave motion. During passage of a wave, the floating rig undergoes a vertical movement. Because the waves are not generally consistent in form or frequency, the rig undergoes a random vertical motion, while procedures such as drilling are carried out on the floating rig.
When drilling of a subsea wellbore is carried out using a floating rig, it is known to provide a riser with a slip joint, which allows the riser to lengthen and shorten as the rig heaves as the sea level rises and falls with the tides and the waves. Such a slip joint is, for example, described in U.S. Pat. Nos. 4,626,135 and 4,411,434, and comprises a first tube section, which is connected to the rig floor, and a second tube section that is connected to the wellhead at the seafloor, where the first and the second tube sections are connected telescopically so that the two tube sections can move vertically with respect to each other, thus permitting vessel heave while maintaining connection of the riser pipe to the seafloor. As the vessel heaves, the slip joint telescopes in or out by the same amount so that the riser below the slip joint is relatively unaffected by vessel motion.
In some configurations, the second tube section comprises an outer tube section that is disposed around the first tube section, which forms an inner tube section. In such configurations, seals are normally provided between the outer and inner tube sections, and these substantially prevent leakage of fluid from the riser whilst allowing the inner tube section to slide relative to the outer tube section.
In the North Sea, a riser is likely to encounter heave of between about 5-10 meters, so the slip joint needs to allow for vertical height movement of at least about 5 meters. Heave of more than 10 meters may be encountered, but it is currently considered unsafe to perform drilling operations under these conditions using existing technologies.
Heave motion at the upper end of the riser leads to a perturbation of flowrate of the drilling fluid out of the top of the riser annulus through the side outlet into the flowline. This means that any attempt to measure the flow rate out of the riser annulus, e.g. by a coriolis flow meter, is flawed and a complicated system of measuring the heave of the floating rig must be used to obtain any meaningful flow rate measurements out of the riser annulus.